1. Field of the Invention
The present invention relates to non-impact printing apparatus for recording using printheads having a plurality of recording elements such as a light-emitting diodes (LEDs) or the like.
2. Brief Description of the Prior Art
In order to reduce the cost of non-impact printheads such as LED printheads, modifications have been proposed to the design of the printheads to reduce the thermal load and current supply requirements of the printhead. In reducing the thermal load on a printhead, savings ensue by reducing the need for more expensive heat sinks and problems associated with nonuniformities in light emissions due to thermal gradients on the printhead may be avoided or reduced. Reduction in current supply requirements reduce the cost of the power supply as well as printhead electrical interconnects. In accordance with one such design a binary architectured printhead is provided with separate enable lines so that different segments of LED's may be enabled at different times during a time period for recording a line of pixels (picture elements). In this printhead all the LED's are arranged in a single straight line across the printhead. A line time for recording a row of pixels in a main scanning direction for a typical use with a photoconductive recording element is approximately 1400 microseconds (.mu.s) (see FIGS. 1a-1d) for an eight-page per minute printer. As noted in FIGS. 1a-1d , the LED's to be enabled for recording during a particular line period are enabled in staggered fashion so that substantially no overlap results in enabling time. In FIGS. 1a-1d, the exposure timing is shown for an LED printhead of 300 dots per inch (dpi) which uses four independent enable lines for four printhead segments of LED's. As the requirement for exposure time per line at 300 dpi in this case is only 200 .mu.s (with a system requiring 3 milliamps per pixel) and the line time for each 1/300th of an inch line spacing is as noted above 1400 .mu.s one can spread out the four strobe enable pulses to have 200 .mu.s exposure durations for each segment without overlapping the exposure times. This concept then reduces the current supply requirement for the printhead.
It is also known to use binary architectured printheads for grey scale recording. In grey scale recording, pixels are created of different densities whereas in binary recording, a pixel is either created or not created. In a binary architectured printhead as the term is used herein, data for determining whether or not a recording element is turned on or off for its next enablement period is determined by a single image data bit. Compare this with a grey level printer architecture, an example of which is illustrated in U.S. Pat. No. 4,750,010, (FIG. 6 thereof) wherein multiple lines for carrying data are required for each LED and multiple data bits are stored on the printhead for each LED before enabling the LED for its next enablement period. One reason for using a binary architectured printhead for grey level recording is that such printers have less data lines and registers for storing data and thus require fewer bonding pads and thereby are cost efficient to manufacture. Additionally, there is demand for such printheads by those incorporating same for use in binary printing applications as well as grey level printing applications, so manufacturing efficiencies based on production quantities may be passed along from the manufacturer of the printhead to the manufacturer of a printer incorporating this printhead. Ultimately, these savings can be passed along to purchasers of the printers.
An example of a binary architectured printhead that is used for grey level recording is described in U.S. Pat. No. 4,525,729. As taught in this patent, a multibit binary-weighted digital word representing a grey level pixel to be printed is shifted to a printhead one bit at a time so that a single bit for each LED is stored on the printhead just prior to enablement of the LED. An exposure clock signal enables those LED's having a respective data bit for a time period related to the binary weight of that data bit. As successive bits are shifted to the printhead, the time durations of the clock pulses changes accordingly until the pixel is printed in accordance with the appropriate digital word representing the pixel's grey level value. Where a pixel is represented by a six-bit digital word, six time periods of exposure are possible and 63 levels of grey, including white, are realizable.
One problem associated with the use of binary architectured printheads for grey level recording is that during recording of the most significant data bit the printhead is drawing considerable current for a relatively lengthy duration thereby causing thermal gradients to arise on the printhead as not all LED's are enabled during every sub-line of recording. The use of staggering, as taught for the embodiment of FIGS. 1a-1d is impractical, as the sub-line used for recording the most significant bit would be too long during recording of the pixels in sub-lines as the image recording medium is moving.
It is therefore an object of the invention to provide for a grey level printer apparatus employing a binary architectured printhead wherein the problems of thermal loading are minimized and exposure periods made relatively shorter than suggested by the prior art.